1. Field of the Invention
The present invention relates to a cooking vessel which may be, and which may be specially intended and designed to be, induction heated. Also included are an alloy for producing such a vessel, and a method for making the vessel.
2. Discussion of the Background
Cooking vessels, such as saucepans, brazing pans or frying pans, are generally made of a food-compatible alloy, such as an austenitic stainless steel or an aluminum alloy. When they are intended to be induction heated, these cooking vessels include, in the external part of their bottom, an insert made of a ferromagnetic alloy. This is because food-compatible alloys are generally nonmagnetic and, consequently, cannot be induction heated given the frequency range (20 to 50 kilohertz) generally permitted. On the other hand, the insert made of a ferromagnetic alloy can be induction heated and constitutes a heating part. Various constructions exist. For example, when the vessel is made of an aluminum alloy, the heating part may be a grid inserted into the bottom of the vessel. When the vessel is made of an austenitic stainless steel, the heating part is a plate made of a ferromagnetic alloy placed on the external wall of the bottom and either separated from the wall by an aluminum layer intended to distribute the heat well or joined to the bottom and covered on its external face with an austenitic stainless steel layer intended to ensure good corrosion resistance.
In general, the ferromagnetic material used for the heating part is a ferritic stainless steel. This technique has the drawback of allowing induction heating up to temperatures that may be as high as 600.degree. C. when the vessel is left on the induction-heating device. As a result, the food may be burnt and the vessels damaged.
In order to remedy this drawback, it has been proposed, particularly in French patent applications No. 2,527,916, No. 2,453,627 and No. 2,689,748, to use heating parts made of a ferromagnetic alloy having a Curie temperature that is not too high, for example between 60.degree. C. and 350.degree. C., so as to avoid overheating. The advantage of a heating part made of a ferromagnetic alloy whose Curie temperature is not too high is that, when the temperature of the heating element becomes close to the Curie temperature of the alloy, the heating part becomes gradually nonmagnetic, which stops the heating. This results in a thermostatic effect whose consequence is that the temperature of the bottom of the cooking vessel cannot exceed the Curie temperature of the alloy of which the heating part is composed. It should be noted however that the aforementioned patent applications either give only very vague information about the alloys that can be used or propose the use of alloys which are not suitable for the envisaged use. For example, French patent application 2,689,748 proposes the use of Fe--36Ni or of Fe--18Ni9Co5Mo or of Fe--80Ni5Mo. None of these solutions are satisfactory.
The alloy Fe--36Ni is well known. This is an alloy having a very low thermal expansion coefficient incompatible with the thermal expansion coefficient of an austenitic stainless steel or of an aluminum alloy. The use of this alloy would result in a very large bimetallic effect leading either to the bottom of the cooking vessel distorting or the heating element disbanding. In addition, this alloy is not corrosion resistant.
The alloy Fe--18Ni9Co5Mo is a well-known maraging steel but does not absolutely have the required properties. Its choice probably results from a confusion between the Curie temperature and the martensitic transformation start temperature Ms.
The alloy Fe--80Ni5Mo, also well known, but it is not suitable either, since its Curie temperature is approximately 450.degree. C.
Moreover, cooking vessels intended for induction heating furthermore have the drawback of producing a disagreeable whistling noise when they are heated.